Abstract A058: Simultaneous characterization of rare immune cell subpopulations and PD-L1-expressing CTCs in peripheral blood of cancer patients

Background: Expression of PD-L1 on tumor and immune markers in tumor tissueis associated with improved response to PD-1 and PD-L1 checkpoint inhibitors. However, the PD-L1 biomarker has limited predictive utility. Multimodal characterization of both tumor and host immune system is an unmet medical need for the improved prediction of immuno-oncology therapy. Given tumor heterogeneity and evolution and temporal changes to the host immune system, metastatic lesions are likely to be undersampled and require a liquid biopsy. We sought to examine the expression of PD-L1 on circulating tumor cells (CTCs) as well as characterize rare immune cell populations with a noninvasive liquid biopsy. Examining dynamic biomarker changes in longitudinal samples could enable the development of novel diagnostic tools for response prediction and pharmacodynamics studies related to immunotherapy. Methods: Blood samples from cancer patients were received and shipped to Epic Sciences. Contrived blood samples were also developed utilizing spike-in of cancer cell line controls into healthy blood samples. RBCs were lysed, and nucleated cells plated onto glass slides utilizing the Epic Sciences process. Slides were stained for nucleus DAPI in addition to IF cocktails and scanned, with targets including CK, CD45, PD-L1, CD4, CD8, Ki-67, LAG-3, and TIM-3. Approximately 3 million nucleated cells were examined through advanced Digital Pathology pipelines to detect and quantify the changes in T-cell populations to infer immune activation, exhaustion, suppression, and circulating tumor burden. Results: Epic’s rare cell detection platform has an analytically validated limit of detection of 1 cell/mL of blood. Three immuno-panels were developed to profile leukocytes subpopulations and CTCs simultaneously. 30 out of 33 lung cancer patients had PD-L1 leukocytes detected, and the incidences of PD-L1+ leukocytes widely ranged from 0 to 0.138% (average 0.0213%, median 0.0118%). PD-L1+ CTCs were observed in the presence of both high and low counts of PD-L1+ leukocyte populations. Conclusions: Epic Sciences CTC platform’s low limit of detection coupled with ability to archive patient blood samples allowed precise quantification of leucocyte subpopulations (CD4 and CD8, etc.) and PD-L1 expression on CTCs retrospectively. Development of a liquid biopsy-based platform that is capable of simultaneously measuring immune biomarkers in CTCs as well as leucocytes will allow real-time assessment and monitoring of response to immune checkpoints inhibitors. Citation Format: Adam Jendrisak, Angel Rodriguez, Nadia Ebrahim, Jiyun Byun, Ryon Graf, Yipeng Wang, Mark Landers, Ryan Dittamore. Simultaneous characterization of rare immune cell subpopulations and PD-L1-expressing CTCs in peripheral blood of cancer patients [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A058.